Thermal transducer with improved time constant



y 1959 w. DJlNlS ET AL I 2,887,556

THERMAL TRANSDUCER WITH IMPROVED TIME CONSTANT Filed Jan. 18, 1956INVENTOR. 38 WILLIAM DJINIS AU L H SAVET F1 '9. 2.

BY v

A TZQPA EY.

i TRANSDUCER WITH IMPROVED TllVIE CONSTANT William Djinis, Syosset, andPaul H. Savet, Westbury, N.Y., assignors to American Bosch AnnaCorporation, 1 a corporation of New York Application January 18, 1956,Serial No. 559,880 i Claims. Cl. 201-63) The present invention relatesto thermal computers and; has particular reference to a new type oftransducer.

[By employing a radically different concept in the design of thetransducer, the time constant has been reduc edby a factor of and theinput power requirements have been reduced by a factor of 100. Also thesire and weight have been reduced materially." However, the circuitapplications of the present transducerare identical with those used in'the past for carryingout the mathematical operations of multiplication,integration, etc. by the mechanism of the transference of thermal energyfrom one element to another. Consequently, as before, the new unit isalso constructed three sets of'wire pairs, viz. one heater pair and twosensor pairs. The principle utilized in the new design, which is notembodied in the former units, is that the time constant is? improved(made smaller) by reducing the thermal capacity of the various elementsto the lowest amount physically possible. Further improvement isachieved both by increasing the rate of transfer of thermal energy fromthe heater to the sensors by using a greater proport on of the emittedenergy (from the heater) to heat the sensors.

' Thethermal capacity is kept small by using extremely finewires foreach heater and sensor. The reduction in thediameter of the wire has atwo-fold effect. Not only is' the mass of the element reduced thereby,but the resistance, which increases as the square of any reductioni'nwire size, is increased. As a consequence, for a given impedancelevel, the length of the wire can be materially shortened, resulting ina still further decrease in the mass of-the element. Thus, the totalmass of the wire will be reduced as the cube of the diameter ratio. Thewires 'ar'e'used with no insulation, since the short lengths permitmounting them firmly with but a small air gap between'them. Thus, theheat capacity previously contribtried by the wire insulation is nowabsent.

The rate of transfer of thermal energy from the heater 1156 the sensorsis increased in several ways. First, the bare wires' are mounted in veryclose proximity to each other so that the thickness of the interveninglayers of 'iiisulating atmosphere is reduced. The heat transported byconduction has a shorter distance to travel. Sec'on'dly,"the transduceris mounted in an evacuated envelope with the result that a higherproportion of the thermal energy is transferred by the radiation process(rather than conduction process) than would be the case were it not inan evacuated envelope. The radiated power being proportional to thefourth power of temperature provides a much higher rate of heat transferthan the conduction process which is proportional to the first power ofthe temperature. The evacuation of the envelope has the added advantageof decreasing the heat capacity ofthe system contributed by the airsurrounding" the wires. Placing the wires close to each other effc'ts'the' condition that the sensors directly intercept a greater portion ofthe energy being emitted by the heater ice due to the larger solid anglecovered by the sensors as viewed from the heater when the wires aremoved in closer. V

A final refinement to the device is achieved by enclosing the spaceoccupied by the fine wires with an infrared reflector to ensure that asmuch as possible of the energy emitted by the heater is effective inraising the temperature of the sensors and, thereby, producing a changein impedance which results in an' output signal voltage. The reflectoris placed as close to the sensitive wires as possible, taking intoaccount the consideration that the wires must not be shorted out whensubjected to mechanical vibration or by the unavoidable slack producedin the wires when heated.

Besides the improvement in the time constant, the new design results infurther advantages which are achieved as a by-product of the newparameters introduced. There is a considerable improvement in theoverall attenuation and stability of the unit, the dimensions of thedevice become small enough to package in an envelope of about the sizeof a sub-miniature tube and, what is of considerable importance, thepower required to drive the input is reduced drastically to the order ofabout 10 milliwatts.

For a more complete understanding of the invention, reference may be hadto the accompanying diagrams, in which Figure 1 shows a preferredconstruction of the thermal element, Figure 2 illustrates a typicaltransducer using the element of Figure 1, Figure 3 is an alternativeembodiment of the invention, and, Figure 4 is yet an other embodiment ofthe invention.

Figure 1 of the drawings shows the preferred construction of a thermalelement embodying the principles which have been found to be necessaryand desirable for a unit having a low time constant. The element isconstructed on an electrically insulating base 10 made preferably of aplastic material such as clear Lucite, for example, in which four pinsor supports 11, 12, 13, 14 are held securely. Since some of thedimensions of the device of Figure 1 are extremely small, the pictorialrepresentation there shown is distorted.

The distance between the facing surfaces of the pins 11 and 12 and pins13 and 14 is one the order of one one-hundredth of an inch and thesespacings are maintained within very close tolerances. The distancebetween pins 11 and 13 and pins 12 and 14 is on the order of two inches.One end of a sensor filament wire 15 is spot welded to a contactterminal 16 at the edge close to pin 11, then bent around pin 11,stretched taut between pins 11 and 13 and then spot welded to theterminal 17 at the edge close to pin 13. Similarly, the other sensorfilament or wire 18 is bent around the pins 12 and 14 and is spot weldedto the contact terminals 19 and 20 at the edges nearest the pins 12 and14. In this fashion, the spacing between the sensor wires 15 and 18 maybe accurately controlled to within the close tolerances and fine spacingfound necessary without extensive assembly problems. The heater filamentor wire 21, located midway between wires 15 and 18, is stretched betweenthe contact terminals 22, 23 and is secured thereto by spot welding. Thewires 15, 18 and 21 all lie in substantially the same plane. Althoughspot welding has been specified throughout, other securing or clampingmeans may be used if convenient. An infra-red reflecting surface orshield 24 of copper or aluminum is bonded to the base 10 and covers thearea between the pins 11, 12 and 13, 14 under the wires 15, 18 and 21.

A cover 25, made of clear Lucite for example, is a channel shaped piecehaving bonded to its central portion a reflecting surface or shield 26as seen where the cover material is broken away. When the cover 25 isfastened to the base 10, as by screws (not shown) which pass through theholes 27 in cover 25 and into tapped holes 28 in the base 10, the wires15, 21, 18 are surrounded entirely by an infra-red reflecting surface.

The sensor wires 15 and 18 are made of a material having a hightemperature coeflicient of resistance drawn to aniextremely'smalldiameter, on the order of inch. The heater wire, of substantially thesame size wire, is made of a material having a low temperaturecoefiicient of resistance. This is in accordance with the principlesunder which earlier thermal units, such as that described in co-pendingpatent application Serial No. 261,255, now Patent No. 2,841,329, filedDecember 12, 1951, operate. However, it will be seen that the long thinwires constitute members of low thermal capacity. In earlier thermalunits the thermal capacity was relatively high and no attempt was madeto reduce the thermal capacity in order to lower the time constant,other more complicated means being used for this purpose. Added rigidityand strength is given to the wire 15, 18 and 21 to prevent shortcircuiting during vibration or elongation of the wires under heating byplacing several small drops or blobs 29 of suitable ceramic on the wires15, 18 and 21. It further protection against vibration is required, theceramic material may be deposited between the wires for their entirelength without departing from the invention.

The shield 24, 26 is an infra-red reflector which ensures that as muchas possible of the energy emitted by the, heater 21 is effective inraising the temperature of the sensors 15, 18. For further increase inthe rate of transfer of heat, the thermal element is preferably enclosedin an evacuated envelope such as shown in Figure 2. In this figure, apair of elements 38, 31 are shown encased in the glass envelope 32,since in practice a pair of elements connected as members of bridgecircuits must nearly always be used. In Figure 2 the common terminal 33of the two elements 30, 31 are connected to the lead in wires 34, oneset of terminals 35 are connected to lead in wires 36 and the other setof terminals 37 are connected to lead in wires 38. A thermal insulatoror barrier 39 separates the two thermal elements 30 and 31. It is knownthat in an evacuated enclosure the transfer of energy is mainly byradiation rather than conduction, and since the radiated power isproportional to the fourth power of temperature the rate of transfer ofheat energy between the heater 21 and the sensors 15 and 18 will begreater than it would be in an unevacuated atmosphere, thereby assistingin reducing the time constant of the device.

Figure 2 is merely illustrative and it will be realized that theevacuated envelope may take other forms, the glass envelope there shown.

The construction shown in Figures 1 and 2 depict a preferred embodimentat the present time. However, it is visualized that many devicesembodying the principles of the invention but physically unsimilar tothe devices of Figures 1, 2 may be made. For example, Figure 3 shows anapparently different device which, however, is constructed with the sameprinciples in mind. The element shown in Figure 3 includes a stack ofsubstantially T- shaped conductors 40, 41 42 and two stacks of L-shapedconductors 43, 44, 45 and 46, 47, 48 all of which are separated fromeach other by an insulating material 49 shown in black in Figure 3.

Heater wires 50 and 51 are stretched between the outer ends ofconductors 44 and 41 and the conductors 47 and 41 respectively and aresecured thereto by welding for example. One pair of sensor wires 52, 53are similarly attached to conductors 40, 43 and 42, 45 respectively andare located on either side of the heater wire 50. Another pair of sensorwires 54, 55 located on either side of heater wire 51 are attached tothe outer ends of conductors 40, 46 and 42, 48 respectively. The entirestructure of Figure 3 may be enclosed in an evacuated envelope, andelectrical connections to the external circuit will be made sistivityand low specific heat.

through leads connected to the lower legs of the conductors 41 through48 as seen in Figure 3. Infra-red reflectors or shields, 56, 57 arelocated in the space behind the heater and sensor wires, i.e., betweenthe extremities of conductors 40, 41, 42 and conductors 43 through 48.

Figure 4 shows yet another possible embodiment of the invention. Here apair of electrically non-conducting discs 61), 61 are separated by asupporting bar 62 which is faced on both sides by infra-red reflectingsurfaces 63, 64. a

Two sensor wires 65, 66 and one heater wire 67 are stretched betweendiscs 60, 61 opposite surface 64 and similarly two sensor wires 68, 69and one heater wire 70 are stretched between discs 60, 61 opposite thesurface 63.

Additional reflecting surfaces which surround the wires 65 through 70are provided, as by the two semi-cylindrical shells 71, 72 shown inFigure 4 which may be attached to the discs 60, 61 or by a tube (notshown) into which the assembly of discs and wires may be inserted.Connections to the external electrical circuit may be 'made through theends of the wires 65 through 70' which pierce the discs 60, 61. Theentire thermal device of Figure;4 may be mounted in an evacuatedenvelope, if desired. 4

Properties of the specific components which are used in the constructionof the physical devices will be enumerated:

The heater wire must have a temperature coefiicient of resistance nearlyequal to zero, should have high re The commercially available wirecalled Evanohm having a diameter of .0009" has been found satisfactoryfor this purpose. I g Y The sensor wire must have as high a temperaturecoeflicient of resistance as possible consistent with resistivity, a lowproduct of specific heat and density, low noise of constant resistanceat any temperature, and moreover must be ductile to allow drawing intoextreme 1y thin wires. For this purpose Balco, ballast nickel orplatinum have been found to be eminently suitable.

The infra-red reflecting surfaces are made of beryllium copper alloy,although other metals such as aluminum ,or copper may be used.

It will be seen that in each of Figures 1, 3, and 4 the heater andsensors are suspended out of contact with anything which might absorbthe heat given off by the heater 21 and thereby add to thermal capacity.There is no insulation on the wires 15, 18, 21 to require heating, andthe dimensions are such that they have minimal mass and thereforeminimal thermal capacity. It should be noted that for a time constant ofof a second, the diameter of the wires should not be greater than onemil. The spacing between the wires should not be greater than 3 mils inorder to keep theattenuation below,,15;. Variation of these parameterswill give different values of time constants and attenuation. a 1

The rate of transfer of energy is increased by theinfrared reflector andby mounting in the evacuated envelope.

We claim:

1. In a thermal transducer, an uninsulated heaterfilament, anuninsulated sensor filament, said sensor and heater filaments being inclose proximity but not in engagement, said filaments being suspendedbetween electrically insulated supports and an infra red reflectoradjacent said filaments for increasing the heat'transfer between saidheater and sensor filaments. 1

2. In a thermal transducer, an uninsulated heater filament, anuninsulated sensor filament, said sensor and heater filaments being inclose proximity but not in engagement, said filaments being suspendedbetween electrically insulated supports and an infra red reflectorsurrounding said filaments for increasing the heat transfer between saidheater and sensor filaments.

3. In a device of the character described, a therrna transducerincluding an uninsulated heater: filament, an uninsulated sensorfilament, said sensor and heater filaments being in close proximity butnot in engagement,

said filaments being suspended between electrically insulated supportsand an infra red reflector adjacent said filaments for increasing theheat transfer between said heater and sensor filaments, said thermaltransducer being enclosed in an evacuated envelope.

4. In a device of the character described, a thermal transducerincluding an uninsulated heater filament, an uninsulated sensorfilament, said sensor and heater filaments being in close proximity butnot in engagement, said filaments being suspended between electricallyinsulated supports and an infra red reflector surrounding said filamentsfor increasing the heat transfer between said heater and sensorfilaments, said thermal transducer being enclosed in an evacuatedenvelope.

5. In a thermal transducer, an uninsulated heater filament, anuninsulated sensor filament, said sensor and heater filaments being inclose proximity but not in engagement, said filaments being suspendedbetween electrically insulated supports and an infra red reflectoradjacent said filaments for increasing the heat transfer between saidheater and sensor filaments, said heaters and sensors being in spacedparallel relation.

6. In a thermal transducer, an uninsulated heater filament, anuninsulated sensor filament, said sensor and heater filaments being inclose proximity but not in engagement, said filaments being suspendedbetween electrically insulated supports and an infra red reflectoradjacent said filaments for increasing the heat transfer between saidheater and sensor filaments, said heaters and sensors being in spacedparallel aligned relation.

7. In a thermal transducer, first spaced supports, a heater filamentstretched taut between said first spaced supports, a plurality of secondspaced supports adjacent but electrically insulated from said firstspaced supports, a sensor filament stretched taut between said secondsupports and positioned parallel to said heater filament, and

an infra red reflector adjacent said heater and sensor filaments forincreasing the heat transfer between said heater and sensor filaments.

8. In a thermal transducer, first spaced supports, a heater filamentstretched taut between said first spaced supports, a plurality of secondspaced supports adjacent but electrically insulated from said firstspaced supports, a sensor filament stretched taut between said secondsup ports and positioned parallel to said heater filament, and aninfra-red reflector surrounding said heater and sensor filaments forincreasing the heat transfer between said heater and sensor filaments.

9. In a thermal transducer, an uninsulated heater filament, a pluralityof uninsulated sensor filaments, said sensor and heater filaments beingin close proximity but not in engagement, said filaments being suspendedbetween electrically insulated supports and an infra red reflectoradjacent said filaments for increasing the heat transfer between saidheater and sensor filaments.

18. In a thermal transducer, first spaced supports, a heater filamentstretched taut between said first spaced supports, a plurality of secondspaced supports adjacent but electrically insulated from said firstspaced supports, a plurality of sensor filaments stretched taut betweensaid second supports and positioned parallel to said heater filament andan infra-red reflector adjacent said heater and sensor filaments forincreasing the heat transfer between said heater and sensor filaments.

References Cited in the file of this patent UNITED STATES PATENTS1,741,231 Grondahl Dec. 31, 1929 2,031,480 Hamada Feb. 18, 19362,663,782 Insley et a1. Dec. 22, 1953

